(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices and more specifically to a method used to process a microlens element for a complimentary metal oxide semiconductor, (CMOS), image sensor device.
(2) Description of Prior Art
Microlens elements are employed as a component of an image sensor device to enhance device sensitivity. The microlens element is formed after formation of the color elements of the image sensor device, via application of a positive photosensitive, transparent organic layer, followed by a patterning procedure employing a photolithographic plate, conventional exposure and development procedures, using an alkaline developer. After definition of the microlens element a baking procedure is used to round the microlens element to increase focusing of incidence light. However rework of the microlens element, if needed as a result of the inability to initially define the desired microlens shape, can present difficulties. First the solvents used to remove the microlens element can damage the underlying color filter elements, or the spacer layer located overlying the color filter elements. In addition the solvents used to remove the microlens shapes can leave unwanted organic residues on the top surface of the spacer layer, subsequently resulting in flattened or mis-shaped, reworked microlens elements, after the bake cycle.
This invention will describe a novel procedure for removing a microlens element shape without damaging underlying materials such as the spacer layer, or the color filter elements. In addition this invention will offer a procedure that will alleviate the effect of any organic residues on the surface to be overlaid by the microlens elements, allowing rounded, microlens shapes be realized after a reflow procedure. Prior art such as Huang et al, in U.S. Pat. No. 6,001,540, describe a process for fabricating a microlens element, however that prior art does not offer the novel rework procedure, nor the novel pre-microlens formation, bake procedure, described in the present invention, allowing rounded, reworked microlens elements to be obtained.
It is an object of this invention to fabricate an image sensor device, featuring a microlens element.
It is another object of this invention to perform a rework procedure, if needed, featuring the removal of a defective microlens element shape from the top surface of the underlying spacer layer, by subjecting the defective microlens shape to a high photolithographic exposure energy, followed by removal of the exposed photoactive compound (PAC), using a base developer solution and de-ionized water.
It is still another object of this invention to perform a bake procedure prior to initiation of another microlens definition procedure, to xe2x80x9cfreezexe2x80x9d or alleviate, the effects of any organic residue, still remaining on the surface of the underlying spacer layer after the base developer removal procedure, allowing a subsequent rounded, reworked microlens element to be formed on the spacer layer, after the reflow procedure.
In accordance with the present invention a method for reworking a defective microlens element, of an image sensor device, is described. After formation of the photodiode components, and the color filter elements, of an image sensor device, a spacer layer is formed on the underlying color filter elements. Application of, and definition of, a photosensitive organic layer results in the formation of microlens elements, located on the top surface of the spacer layer. Prior to a reflow procedure, used to round the microlens elements, and to increase focusing properties of this element, if the desired shape of the pre-reflowed microlens element has not been obtained, a rework procedure is initiated. First a high energy exposure is used to convert the photoactive compound (PAC), microlens shape to an acid containing organic shape, followed by removal in a base developer solution. Next a frozen bake procedure is performed to cure, or freeze, any organic phororesist residuals, still remaining on the surface of the spacer layer after the base developer removal procedure. Application of, and definition of another photosensitive organic layer results in the attainment of the reworked microlens element shape. A reflow procedure is then employed to create the desired microlens, rounded shape.